The present invention relates to an energy conversion system, and more particularly, to an improved thermal-mechanical device utilizing a material having a memory effect.
Various devices are known utilizing NITINOL, a near stoichiometric nickle-titanium alloy (as disclosed in U.S. Pat. No. 3,174,851) to convert thermal energy into mechanical energy. NITINOL exhibits a "memory effect" in various forms by heating the alloy from a temperature below its transition temperature range to a temperature above its transition temperature range. There are two principle ways in which the NITINOL memory effect and its associated force can be used, namely, in the unit directional mode to cause the elongated material to contract and in the bending mode to cause the bent material to regain its original shape.
The most practical of the known devices for utilizing this memory effect of NITINOL is disclosed in U.S. Pat. No. 4,275,561 to the present inventor. The device utilizes the memory effect of a material such as NITINOL in the bending mode by bending it about a relatively hotter wheel which is at a temperature above the transition temperature range of the memory effect material. Thermal energy is transferred from the relatively hotter wheel to the wire over at least the major portion of the circumferential contact of the memory material with the relatively hotter wheel. A larger, relatively cooler wheel is drivingly connected to the relatively hotter wheel by at least one NITINOL transmission element in the form of a wire. Both the hotter wheel and the larger, cooler wheel are mounted for rotation about fixed axes.
Because the relatively hotter wheel is kept at a temperature above the transmission temperature range of the memory effect material, the NITINOL transmission element while bent around the wheel is heated and straightens due to the memory effect. The patent discloses that the relatively hotter wheel can be hollow to permit the flow therethrough of a heated fluid medium. Since the relatively hotter wheel rotates, a sealing, bearing means must be utilized to be able to introduce the heated fluid medium thereinto while permitting the wheel to rotate. Generally, the relatively hotter wheel is supported on a hollow shaft with the hollow shaft itself rotatably supported in sealed bearings to provide a rotatable, sealed connection between the stationary tubular elements serving as an input pipe and output pipe and the rotating input and output sections formed by the hollow shaft. This construction permits into-pumping of the heated fluid medium. However, the necessity of using a sealing, bearing means produces certain problems in the practical construction of a useful device in accordance with this patent in certain uses.
These problems include the possibility of leakage of the heated fluid medium through the sealing, bearing means, the necessity for providing a complex system for lubrication of the bearings, energy losses in the bearing structure which not only is a rotational bearing but also is a seal for the heated fluid medium, and the like.
Further, as higher speeds of rotation of the transmission element made of the memory-effect material are obtained, the problem of cooling the NITINOL transmission element around the relatively cooler wheel becomes more difficult. Some form of cooling means may be necessary which further complicates the structure of the device. That is, cooling means may be necessary because as higher speeds are obtained, the NITINOL transmission element has less time to cool down below its transitional temperature from its contact with the relatively hotter wheel and thus, less thermal energy is lost to the environment. The inability of dissipating heat from the transmission element which is moving faster and faster becomes self-limiting, thereby affecting power output of the device.